The present invention relates to the manufacturing of printed circuit boards and more particularly to the manufacturing of a printed circuit board in which nickel is used as the etch resist in forming the electrical conducting areas of the board.
A printed circuit board is employed as a low cost vehicle for interconnecting various components in an electrical circuit. A simple printed circuit board can be made by applying a resist material to a copper layer that is laminated to a fiberglass core so as to define circuit patterns and chemically etching away the unwanted copper layer. The particular method of applying the resist, the type of resist, and the type of etchant are process details that are dictated by the end requirement of the printed circuit board and the facilities that are available. In the manufacture of the printed circuit boards, holes are drilled through the board and copper is then plated into the hole for interconnecting the leads of circuit components located on the boards prior to a mass soldering operation which may be accomplished in a conventional manner such as wave soldering of the circuit board. The most commonly used solder is a tin-lead eutectic alloy of 63% tin and 37% lead which has a relatively sharp melting point of around 361 degrees F. The operating temperature for associated wave soldering is somewhere in the range of 425 degrees-525 degrees F., with 490 degrees F. being optimum. Poor solderability can take place at lower temperatures. Higher soldering temperature can damage heat sensitive components, can cause board warpage and/or cause excessive oxidation of molten solder. During the wave soldering operation, undesirable side effects that may occur include excessive solder consumption, which increases the weight of the wiring board when large ground plane areas thereof become coated with solder, and a tendency for the solder to bridge between circuit pads and traces as well as between the adjacent circuit traces. In order to reduce the amount of solder that is applied to circuit areas such as ground planes and traces and to reduce the tendency for solder bridging, and to protect electrical circuitry from contamination, a solder mask or solder resist is often applied over bare copper circuit traces and bare ground planes. This resist is an organic coating that is applied over the entire board, except for windows around areas where a solder joint is required such as in circuit pads and connecting fingers. This mask works well over bare copper traces and ground planes.
In order to improve the solderability of circuit pads and holes, the practice is to precoat them with solderable metal. The most desirable material for this application is an alloy of tin and lead which is similar to the eutectic alloy that is actually used in the subsequent wave soldering operation. The tin-lead coating is generally electroplated onto the pads and in the holes and then reflowed. It may also be applied as a solder dip coating. Although other electro-deposited metals such as tin, nickel, tin-nickel alloy and even gold may be used to cover the copper layer-traces, solder plate is preferred due to its compatibility with the solder material used in wave soldering and the fact that it will melt during wave soldering and cause a liquid-to-liquid contact. Tin is next in preference with a melting point of only 450 degrees F. Tin-nickel does not solder well and has a very high melting point which is much greater than that of tin-lead. Gold is cost prohibitive and a severe contaminant in the wave solder process, as well as causing brittle solder joints. Nickel provides strength to the traces but has poor solderability.
In the past, when a tin-lead solder overplating was used as an etch resist for the copper traces and ground planes and had a solder mask applied over it, the solder under the mask would melt and flow sufficiently during the wave soldering operation to cause bridging of circuit traces under the solder mask as well as wrinkling and/or rupture of the solder mask itself. This wrinkling is unattractive and subjects the mask to peeling and cracking which allows acid based flux to attack the circuit patterns and reduces the mask's effectiveness as a conformal coating that is employed to protect the circuit from the elements; that is, high humidity and corrosive atmosphere. The basic problem then is to produce a circuit board that solders well and has a solder mask that will not wrinkle during a wave soldering operation and has traces that will not bridge under the solder mask.
One approach is to apply the solder mask over bare copper (SMOBC) traces and ground planes. Another prior art technique is to chemically remove any tin-lead solder overplating from copper traces and ground planes prior the application of the solder mask. This adds the cost of an additional removal operation to the manufacture of the circuit board as well as the problem of removing lead from the waste products. Another prior art technique is to overplate the copper of the circuit patterns with a high melting point metal, i.e., a tin-nickel alloy or nickel itself, and then selectively plate tin-lead solder on only the areas of holes and pads, a solder mask then being applied over the tin-nickel on the traces and ground planes. Since the melting point of the nickel or tin-nickel overplate is much greater than that of the molten solder in the wave soldering operation, the nickel or tin-nickel does not melt and there is no deformation of the solder mask. Another prior art technique is to use nickel as an etch resist and then chemically activate the nickel to allow solder to adhere thereto. This is because when nickel has been used as an etch resist, it has been found that a condition of poor solderability occurred when circuit components were attached to the circuit board by mounting the components to the pads and the plated-through holes in the board. Examples of these prior art techniques may be found in U.S. Pat. Nos. 3,704,207; 4,024,631; 4,088,545; 4,104,111; and 4,487,654.